The present invention relates to a mobile radio communication system and, more particularly, to a radio channel control system for a small-zone mobile communication system, e.g. a portable telephone system or a cordless telephone system.
Radio channel control methods known in the mobile communications art may geneally be classified into two types, i.e., a centralized control type in which a base station collectively supervises channels which are assigned to a group of mobile stations associated therewith, and a decentralized control type in which each of mobile stations selects an available channel independently of the others. A personal radio system is one of appications of the decentralized control type method. A domestic maritime mobile radiotelephone system installed by NTT (Japan) is another application which employs a multi-channel cyclic switching principle, i.e., assigns no exclusive common control channels and allows pilot signals to be sent on desired channels.
The channel control for the above-mentioned personal radio system is performed using exclusive common control channels. The problem with such a manner of channel control is that where base stations are so distributed as to provide continuity among different zones as viewed in a plane such as for portable telephones, cochannel interference occurs in overlapping areas of nearby zones where the desired-to-undesired signal (DU) ratio is comparatively low, making it difficult to set up a call. The cochannel interference has customarily been coped with by providing each mobile station with a carrier sensing capability, which allows the mobile station to sense presence/absence of a radio wave.
Meanwhile, the radio channel control elaborated for the domestic maritime mobile radiotelephones assigns different channels to different zones because the zones are each comparatively wide. This successfully eliminates the cochannel interference. However, in a system in which the individual zones are very small such as a portable telephone system or a cordless telephone system, it is significantly difficult to set up a channel arrangement which promotes efficient use of frequencies partly because each zone is not circular but rather linear and partly because base stations cannot be located in adequate places.